Incandescent and fluorescent light sources have been the mainstay of traditional lighting for residential and commercial applications for many decades. Unfortunately, these light sources have inherent energy inefficiencies, which have driven a search for improved light sources as the “green revolution” has progressed.
Because solid-state lighting (SSL) sources promise improved energy efficiency, they have become attractive alternatives to traditional lighting. The transition to SSL has been projected to reduce global electricity used for lighting by 50% by the year 2025. In addition, unlike most traditional lighting sources, the emission wavelengths from many SSL light sources can be controlled to effect a desired “color temperature.”
Color temperature refers to the color characteristics of light. Light whose hue tends toward yellow (e.g., that emitted by tungsten-based household incandescent lamps) is typically considered “warm,” while light having a hue that is more toward blue (e.g., that emitted by conventional fluorescent lamps) is typically considered “cool.” The Color Rendering Index (CRI) is a scale, from 1 to 100, that is based on the ability of a light source to accurately render all colors of its spectrum (as compared to a “perfect” black-body light source). The lower a source's CRI rating, the less accurately colors will be reproduced by that source. Normally, a CRI of at least 90 is considered desirable, and indoor light applications typically require CRI values of at least 80.
Conventional SSL sources that are based on rare-earth-doped phosphors (e.g., Ce3+-activated yellow phosphors, etc.) are excited into emission by absorption of light from blue-wavelength light-emitting diodes (LEDs). These sources generally have poor color rendering properties because they employ phosphors that do not emit across the entire visible light spectrum. Such prior-art phosphors typically lack a red component in their output light; therefore, they produce a white light that appears cold to the eye (i.e., its hue is more “bluish”). CRI indices greater than 90 have been realized in multi-phosphor systems, but these systems generally suffer from poor efficiency due to self-absorption. In addition, the different individual phosphors degrade at a different rates, leading to white light that becomes colored over time. Further, single and mixed phosphors require synthesis at high temperature (typically 1300-1900° C.), making their production complex and expensive.
To circumvent these problems, there has been recent emphasis on research into single-phase broadband white-light emitting phosphors suitable for SSL sources. Two-dimensional, cadmium-sulfide/selenide materials offer some promise and examples of such phosphors are described by W. Ki, et al., in “A Semiconductor Bulk Material That Emits Direct White Light,” J. Am. Chem. Soc., Vol. 130, page 8114 (2008). These materials demonstrated that a photoluminescence quantum efficiency (PLQE) of 4-5% could be achieved. The inclusion of cadmium, however, is generally considered undesirable due to its toxicity.
It has been shown that, by using substitution chemistry, Mn2+ doping, and crystal engineering, PLQE can be improved by up to 37%, as reported by M. Roushan, et al., in “Solution-Processable White-Light-Emitting Hybrid Semiconductor Bulk Materials with High Photoluminescence Quantum Efficiency,” Chem., Int. Ed., Vol. 51, page 436 (2012). Unfortunately, while these results show progress toward a suitable white-light-emitting phosphor, the reported materials are expensive to synthesize, lack highly defined structure, and include materials that are toxic to humans.
Other emerging SSL technologies include cadmium-selenide nanocrystal-based sources. Nanostructures characterized by high surface-to-volume ratio (e.g. 1.5-nm quantum dots) have shown white-light emission due to surface states that lead to sub-band-gap electronic states. Quantum-dot sources based on this technology have shown excellent color-rendering properties. Unfortunately, particle aggregation can quench emission; therefore, a dispersive polymer is typically required to prevent such aggregation. Most polymers, however, tend to degrade rapidly under constant exposure to ultraviolet light, thereby limiting the lifetime of these sources. In addition, as mentioned above, the toxicity of cadmium represents an additional disadvantage in such devices. Further, in order to realize nanocrystals having precise size, complex processing techniques are needed making these sources expensive.
To date, a viable single-phase, low-cost, long-term-stable, white-light-emitting phosphor has yet to be realized.